This invention relates to a fabrication process for making thin-film transistors.
Thin-film transistors using amorphous silicon (a-Si) are characterized by a high switching ratio and can be fabricated by a low-temperature process employing a glass substrate. They are used in devices such as active-matrix liquid-crystal displays. FIGS. 2A, 2B, and 2C are sectional views showing the steps in their fabrication.
The initial steps are illustrated in FIG. 2A. A first metal layer of a material such as nichrome (NiCr), chromium (Cr), or tungsten (W) on the order of 100 .ANG. to 5000 .ANG. in thickness is formed by vacuum evaporation or sputtering on a dielectric substrate 11 such as a glass substrate, and is patterned to form a gate electrode 12. Next a glow discharge step using NH.sub.3 and SiH.sub.4 as the principal gases is carried out to cover the entire substrate 11, including the gate electrode 12, with a silicon nitride film (SiN.sub.x) having a thickness on the order of 0.1um to 1.0um, which serves as a gate insulation layer 13. Then a glow discharge step using SiH.sub.4 as the principal gas is carried out to form an a-Si semiconductor layer 14 having a thickness on the order of 0.01 .mu.m to 1.0 .mu.m on top of the gate insulation layer. The glow discharge steps that create the gate insulation layer 13 and the a-Si semiconductor layer 14 are performed continuously in the same fabrication apparatus without breaking the vacuum.
Next the a-Si semiconductor layer 14 and the silicon oxide film 13 are patterned by a photolithography and dry etching step, specifically by a plasma etching step using CF.sub.4 +O.sub.2 gas, leaving the element region as shown in FIG. 2B. After this, a second metal layer 15 consisting of aluminum (A1) having a thickness on the order of 0.2 .mu.m to 2.0 .mu.m is formed by vacuum evaporation on top of the a-Si semiconductor layer 14.
This second metal layer 15 is then patterned as shown in FIG. 2C to form a drain electrode 15a and a source electrode 15b on the a-Si semiconductor layer 14, thus completing the a-Si thin-film transistor. A passivation layer, not shown in the drawings, may be formed over the completed device for surface protection.
Forming the silicon nitride gate insulation layer 13 and the a-Si semiconductor layer 14 continuously in the same apparatus inherently requires an apparatus which can be used for both process steps. Such an apparatus is not always available. Moreover, even when such an apparatus is available, the quality of the interface between the gate insulation layer 13 and the a-Si semiconductor layer 14 formed by the above process is not sufficient for some applications. Thin-film transistors fabricated as described above have a limited switching ratio (I.sub.on /I.sub.off ratio) and electron mobility (.mu.), and an undesirably large threshold voltage (V.sub.T). The transistors must therefore be relatively large in size. Liquid-crystal displays using such transistors have been unable to achieve a high display resolution.